At this point we know that Webpack is dead. Unless there is a new release that rewrites it in Rust or Go and makes it comparable in speed to Vite/SWC which will never happen. The two different approaches seem to be either going with SWC or Vite. I'm honestly not sure which one will prevail, or if they will both co-exist, splitting the previous market share taken up by Webpack.
But, vite is working on to become the next nextjs, like ssr, isg etc. Nextjs only support react, so it's easier to leverage the react's ability to do these.
When there is ssr, there is runtime, more than just a dev/build tool.
It sounds like you might have some misunderstanding. You can write SSR applications with Vite, but each framework or application is responsible for the rendering. There isn't really such a thing as an SSR runtime in Vite. It will run code on the development server in SSR mode, but all it does is make sure it's picking up code updates and passes it through to Node.js.
(With that said, I'd love to see Next use Vite, but iirc the webpack dev works for Vercel [Nextjs parent] now)